Time-of-Flight Mass Spectrometry (“TOFMS”) is a commonly performed technique for qualitative and quantitative chemical and biological analysis. Time-of-flight mass spectrometers permit the acquisition of wide-range mass spectra at high speeds because all masses are recorded simultaneously. As shown in FIG. 1, most time-of-flight mass spectrometers operate in a cyclic extraction mode and include primary beam opics 7 and time-of-flight section 3. In each cycle, ion source 1 produces a stream of ions 4, and a certain number of particles 5 (up to several thousand in each extraction cycle) travel through extraction entrance slit 26 and are extracted in extraction chamber 20 using pulse generator 61 and high voltage pulser 62. The particles then traverse flight section 33 (containing ion accelerator 32 and ion reflector 34) towards a detector, which in FIG. 1 consists of micro-channel plate (“MCP”) 41, anode 44, preamplifier 58, constant fraction discriminator (“CFD”) 59, time-to-digital converter (“TDC”) 60, and computer (“PC”) 70. Each particle's time-of-flight is recorded so that information about its mass may be obtained. Thus, in each extraction cycle a complete time spectrum is recorded and added to a histogram. The repetition-rate-of-this-extraction cycle is commonly in the range of 10 Hz to 100 kHz.
If several particles of one species are extracted in one cycle, then these particles will arrive at the detector within a very short time period (possibly as short as 1 nanosecond). When using an analog detection scheme (such as a transient recorder in which the flux of charge generated by the incoming ions is recorded as a function of time), this near simultaneous arrival of particles does not cause a problem because analog schemes create a signal that is, on average, proportional to the number of particles arriving within a certain sampling interval. However, when a counting detection scheme is used (such as a time-to-digital converter in which individual particles are detected and their arrival times are recorded), the electronics may not be able to distinguish particles of the same species when those particles arrive too closely grouped in time. (A single signal is produced when a particle impinges upon the counting electronics. The signal produced by the detector is a superposition of the single signals that occur within a sampling interval.) Further, most time-to-digital converters have dead times (typically 20 nanoseconds) that effectively prevent the detection of more than one particle per species during one extraction cycle.
For example, when analyzing an air sample with twelve particles per cycle, there will be approximately ten nitrogen molecules (80% N2 in air with mass of 28 amu) per cycle. In a time-of-flight mass spectrometer having good resolving power, these ten N2 particles will hit the detector within two nanoseconds. Even a fast TDC with a half nanosecond bin width will not be able to detect all of these particles. Thus, the detection system will become saturated at this intense peak. FIG. 2 shows these ten particles 6 impinging upon a detector consisting of electron multiplier 41 (with MCP upper bias voltage (75) and MCP lower bias voltage (76) as indicated), single anode 44, preamplifier 58, CFD 59, TDC 60, and PC 70. (MCP 41 in FIG. 2 consists of two chevron mounted multichannel plates. As would be apparent to one of skill in the art, circuitry would also be included to complete the electrical connection between the upper and lower plates. This additional circuitry is not shown in the figures.) TDC 60 will register only the first of these ten particles. The remaining nine particles will not be registered. Because only the first particle is registered, peaks for the abundant species (N2 and O2) will be artificially small and will be recorded too early, resulting in an artificially sharpened peak whose centroid is shifted to an earlier and incorrect time of flight. These two undesirable effects—incorrect intensity and artificially shortened time of flight—are referred to as anode/TDC saturation effects. These anode/TDC saturation effects are therefore different from the electron multiplier gain reduction (sometimes called multiplier saturation) that occurs when too many ions impinge the electron multiplier so that the electron multiplier is no longer able to generate an electron flux that is proportional to the flux of the incoming ions.
In an attempt to overcome anode/TDC saturation effects, some detectors use multiple anodes, each of which is recorded by an individual TDC channel. (An anode is the part of a particle detector that receives the electrons from the electron multiplier.) FIG. 3 shows such a detector with a single electron multiplier 41 and four anodes 45 of equal size. Each of the four anodes is connected to a separate preamplifier 58 and CFD 59. Each of the four CFDs is connected to IDC 60 and PC 70. This configuration permits the identification of intensities that are four times larger than those obtainable with a single anode detector. However, even with four anodes, the detection of the ten N2 particles 6 leads to saturation since on average there will still be more than one particle arrival per anode. In principle, anode/TDC saturation could be avoided entirely by adding even more anodes. However, this solution is complex and expensive since each additional anode requires its own TDC channel.
Instead of using multiple anodes that each receive the same fraction of the incoming ions, one may use multiple anodes in which each anode receives a different fraction of the incoming ions. (The anode fraction is the fraction of the total number of ions that is detected by a specific anode.) By appropriately reducing this fraction, anode/TDC saturation effects can be reduced. See, for example, PCT Application WO 99/67801A2, which is incorporated herein by reference. One way to provide anodes that receive different fractions of the incoming ions is to provide electron multiplier 41 followed by anodes of different physical sizes as shown in FIG. 4, in which large anode 46 is located adjacent to small anode 47. As before, each anode is connected to a separate preamplifier 58 and CFD 59, and the CFDs are connected to TDC 60 and PC 70. In the example of FIG. 4, two unequal sized anodes are provided having a size ratio of approximately 1:9. As a result, the small anode detects only one N2 particle per cycle, which is just on the edge of saturation. Less abundant particles such as Ar (1% abundance in air and thus 0.12 particles per cycle) are detected without saturation on the large anode. Thus, with two anodes of unequal size it is possible to increase the dynamic range by a factor of approximately ten or more. A multi-anode detector with equal sized anodes would require ten anodes to obtain the same improvement.
In theory, the dynamic range of the unequal anode detector can be further reduced by further decreasing the size of the small anode fraction or by including additional anodes with even lower fractions. However, this theoretical increase in dynamic range is prevented by the presence of crosstalk from the larger anodes to the smaller anodes. In typical multi-anode detectors, the crosstalk from one anode to an adjacent anode ranges approximately from 1% to 10% when a-single ion hits the detector. Thus, if 10 particles are detected simultaneously on a large fraction anode, the crosstalk to an adjacent small fraction anode may range from 10% to 100%. In such cases the small anode would almost always falsely indicate a single particle signal.
Bateman et al. (PCT Application WO 99/38190) disclose the dual stage detector shown in FIG. 5 where anode 47, in the form of a grid or a wire, is placed between MCP electron multipliers 41 and 50. However, instead of distributing different fractions of the incoming ion events (i.e., incoming particles 6) among different anodes, the detector of FIG. 5 distributes the secondary electrons of each ion event. They consider anode 47 to be the anode on which saturation effects are impeded. If anode 47 is a 10% grid, then anodes 47 and 46 each receive the same number of ion signals. The ion signals on anode 46, however, are larger (on average) because of the additional amplification provided by MCP 50. This type of additional amplification is useful in an analog acquisition scheme or in a combined analog/TDC acquisition system, in which the same principle has been used with dynode multipliers. However, in a pure TDC (or counting) acquisition system, increasing the dynamic range with two anodes of equal signal rates, but unequal signal sizes, is quite difficult.
Bateman et al. also suggest using different threshold levels on discriminators 59 to achieve different count rates on the two anodes. This suggestion, however, makes the detection characteristics largely dependent on the pulse height distribution of the MCPs. Also, the same technique could be applied with a single gain detector. Further, placing the small anode between the MCP and the large anode results in extensive crosstalk from the large anode to the small anode.
An object of the present invention is to provide a method and apparatus for reducing crosstalk and increasing dynamic range in multiple anode detectors. That is, an object of the present invention is to reduce crosstalk from anodes receiving a larger fraction of the incoming ions to those anodes that receive a smaller fraction of the incoming ions, thereby reducing the occurrence of false signals on the small fraction anode. A further object of the present invention is to provide a minimum variance procedure for combining—either in real time or off line—the counts from the separate anodes. A further object of the present invention is to provide a detector and associated electronics that will combine the signals from any mixture of small and large anodes to achieve a real time correction of ion peak intensity and centroid shift. A further objective of the present invention is to extend the dynamic range of a multi-anode detector by providing multiple electron multiplier stages where the electron multiplier gain reduction that occurs after the first stage is minimized in subsequent stages.